As described in the U.S. Army Engineering Design Handbook on Recoilless Rifle Weapon Systems (AMCP 706,238), all recoilless rifles use a venturi behind the breech section thereof to vent propellant gases. This venturi is generally cone-shaped and is designed so that the momentum of the escaping propellant gases will equal that imparted to the projectile, thus ensuring recoilless operation. The venturi must be designed to withstand hoop loads resulting from the static pressure of the gases, as well as an axial or thrust load which is a function of the cone divergence angle.
The major in-service recoilless rifles (the U.S. 90 mm and 106 mm Systems and the Swedish 84 mm Carl Gustaf) all use machined steel venturis which are relatively heavy and costly to fabricate given their small wall thickness and three-dimensional configurations. For example, the venturi section of a typical 84 mm in-service recoilless gun, including the venturi cone and the breech adapter portion therefor weighs about 2.0 kg. Any improvement in that figure is a bonus to the infantryman who has to carry his gun over long distances.
Until recently the venturi design has not received as much attention as the barrel portion of the firearm; see for example the copending application referred to above. The barrel section is more important to the proper functioning of the firearm and there has been an absence of major problems with the venturi. However, significant weight savings in the barrel section through the use of a carbon-fibre/epoxy resin composite material (from 8.5 kg to 2.8 kg) have resulted in the venturi portion assuming a larger percentage of the overall weight and it is therefore advantageous to endeavour to reduce the weight of the venturi portion.
A prime consideration in redesigning the venturi portion was to ensure that there is sufficient gas erosion resistance in the critical breech area. It was therefore decided to retain the throat ring/breech adapter of the existing firearm and to concentrate on the cone portion of the venturi for weight reduction. Furthermore, the throat ring adapter is subjected to complicated loadings on firing and it was deemed desirable to continue with an accepted, suitable design.
The decision to use the existing throat ring adapter complicates, however, the problem of reducing venturi weight since in the existing venturi the adapter constitutes 38.2% of the total weight. In a reduced-weight venturi it would represent a higher percentage of the total weight. In attempting to redesign the venturi cone the advantages achieved with filament wound barrels, as documented in the copending application referred to above, were deemed to be significant and consequently it was decided to apply the knowledge gained in the barrel development work to the venturi situation. It was not possible, however, to prepare a venturi cone in the same manner as a barrel section because they are subjected to different loadings and design requirements. For example, the transient thrust loads experienced by the venturi cone have a profound effect on the threaded connection between the cone and the throat ring/breech adapter. Also, the cone portion is more prone to impact than is the barrel section and that fact must also be taken into consideration when choosing materials and winding angles.